The human retina is susceptible to damage from a variety of environmental factors, including laser light impact and other trauma, as well as disease. Once damaged, the cells responsible for capturing light energy and transducing it into a neural signal, the photoreceptors, do not regenerate. In fact, none of the neural cells of the retina can yet be made to readily regrow in the adult human. When damage is severe enough, there is permanent vision loss in an area. Healthy photoreceptors do not migrate long distances toward the damaged area to replace damaged ones.
If the affected region is in the central macula, known as the fovea, then the ability to see fine detail, read at rapid rates, or recognize objects at great distances may be lost. The peripheral areas of vision do not have sufficient sampling density to perform these tasks to the same degree. Thus, early detection and treatment of potentially sight-robbing damage are crucial in maintaining central vision.
One of the chief problems in early detection has been the difficulty of imaging damage to a small area of retina. The macula presents a small target, 6000 microns. The portion that is necessary for seeing damage that precludes observation of fine detail and reading is even smaller, about 600 microns. To examine this latter portion properly, it would be desirable to image the central 20 degrees of the macula with sufficient magnification and contrast to determine whether an individual is at risk for permanent vision loss.
The ophthalmoscope or fundus camera has been used to view and image the retina. Originally, these devices flooded the retina with white light. Subsequent devices have used selective wavelengths that have been found suitable for viewing or imaging particular structures or contrast between structures.
Flood illumination produces images of the retina that often are subject to poor contrast due to the long range scatter not only from out of plane tissues, but also from the biological tissues that are inherently scattering, especially those within and near the retina. Scanning of an illumination source is a well-known method to increase the contrast in images by reducing unwanted scatter. Webb et al. disclose double scanning optical apparati that scan both incident and reflected light using a horizontal scanning element, such as a rotating multifaceted polygonal reflector, and a vertical scanning element, such as a reflecting galvonometer. The instrument is able to provide a two-dimensional output representative of reflection characteristics of the eye fundus. See U.S. Pat. Nos. 4,768,873 and 4,764,005. Webb et al. disclose a laser scanning ophthalmoscope in which a line beam is scanned across an eye. See U.S. Pat. No. 4,768,874.
Reflectometry techniques with a scanning laser ophthalmoscope (SLO) have been described by Elsner et al. See, for example, Elsner A. E., et al., Reflectometry with a scanning laser ophthalmoscope, Applied Optics, Vol. 31, No. 19 (July 1992), pp. 3697-3710. The SLO is advantageous for reflectometry, i.e. quantitative imaging, in that a spot illumination is scanned in a raster pattern over the fundus, improving image contrast significantly over flood illumination. Unwanted scattered light can be further rejected by using confocal apertures. The aperture may be a circle of variable diameter or annular, depending on the desired mode. The desired light is transmitted to a detector.
The extensive use of near infrared light as an illumination source, in lieu of other wavelengths or color images, is further discussed in Elsner, A. E., et al., Infrared Imaging of Sub-retinal Structures in the Human Ocular Fundus, Vision Res., Vol. 36, No. 1 (1996), pp. 191-205; Elsner, A. E., et al., Multiply scattered light tomography: Vertical cavity surface emitting laser array used for imaging subretinal structures, Lasers and Light in Ophthalmology, 1998; Hartnett, M. E. and Elsner, A. E., Characteristics of Exudative Age-related Macular Degeneration Determined In Vivo with Confocal and Indirect Infrared Imaging, Ophthalmology, Vol. 103, No. 1 (January 1996), pp. 58-71; and Hartnett, M. E., et al., Deep Retinal Vascular Anomalous Complexes in Advanced Age-related Macular Degeneration, Ophthalmology, Vol. 103, No. 12 (December 1996), pp. 2042-2053. Infrared imaging with a scanning laser ophthalmoscope (SLO) has been used to perform reflectometry techniques to view the eye rapidly and noninvasively. Once implemented with scanning laser devices, infrared and near infrared imaging of sub-retinal structure in the ocular fundus has been able to reveal sub-retinal deposits, the optic nerve head, retinal vessels, choroidal vessels, fluid accumulation, hyperpigmentation, atrophy, and breaks in Bruch's membrane. Infrared light is absorbed less than visible light and may scatter over longer distances. With flood illumination, these features have not been observed with the same clarity or in lesser numbers. The relatively less absorption has advantages in that a minimum of light may be used as an illumination source. However, the reflected and scattered light must be separated in some manner, and the light used to accentuate the features of interest made available to the user.
The methods for detecting and localizing such features are described in the prior art of the inventor and colleagues: Elsner, A. E., et al., Infrared Imaging of Sub-retinal Structures in the Human Ocular Fundus, Vision Res., Vol. 36, No. 1 (1996), pp. 191-205; Elsner, A. E., et al., Multiply scattered light tomography: Vertical cavity surface emitting laser array used for imaging subretinal structures, Lasers and Light in Ophthalmology, (1998); Elsner, A. E., et al., Foveal Cone Photopigment Distribution: Small Alterations Associated with Macular Pigment Distribution, Investigative Ophthalmology & Visual Science, Vol. 39, No. 12 (November 1998), pp. 2394-2404; Hartnett, M. E. and Elsner, A. E., Characteristics of Exudative Age-related Macular Degeneration Determined In Vivo with Confocal and Indirect Infrared Imaging, Ophthalmology, Vol. 103, No. 1 (January 1996), pp. 58-71; and Hartnett, M. E., et al., Deep Retinal Vascular Anomalous Complexes in Advanced Age-related Macular Degeneration, Ophthalmology, Vol. 103, No. 12 (December 1996), pp. 2042-2053, as examples. Specifically, when a retinal image acquired is only of the macula, centered on the fovea, the only features present with near infrared illumination are the normal retinal and choroidal blood vessels, and potentially superficial reflections, such as from the fovea. In a monochromatic image, any differences from these features in image intensity, beyond the noise inherent in any electronic signal, is interpreted as pathology. If the optic nerve head is also in the image, either due to a sufficiently large field of view or positioning of the eye with respect to the instrument to incorporate this feature, then intensity changes in the retina also define the position and condition of such a structure. It has been shown by Hartnett and Elsner, 1996, and Miura et al, 2002, that such monochrome images using infrared illumination are superior for the detection of certain features over methods using color photography. Hartnett, M. E. and Elsner, A. E., Characteristics of Exudative Age-related Macular Degeneration Determined In Vivo with Confocal and Indirect Infrared Imaging, Ophthalmology, Vol. 103, No. 1 (January 1996), pp. 58-71; Miura, M., et al., Grading of Infrared Confocal Scanning Laser Tomography and Video Displays of Digitized Color Slides in Exudative Age-Related Macular Degeneration, Retina, Vol. 22, No. 3 (2002), pp. 300-308. No modestly priced, simple to use instrument is available that operates using this method.
Current prior art retinal imaging instruments use a photodiode, a photomultiplier tube, or other spot detector, i.e. single point detection. In previously published retinal imaging devices, single spot detectors have been placed optically in the retinal plane in the instruments above (Webb, as above) or the pupil plane (van Norren). Norren, D. and J. van der Kraats, A continuously recording retinal densitometer, Vision Research 21 (1981), pp. 897-905. Koester et al. have described a confocal microscope using a slit scanning system in studying the corneal endothelial cell layer of the eye, as well as the ear. Koester, C. J., et al., Optical sectioning with the scanning slit confocal microscope: applications in ophthalmology and ear research, SPIE: New Methods in Microscopy and Low Light Imaging, Vol. 1161 (1989), pp. 378-388. Koester, C. J., Scanning mirror microscope with optical sectioning characteristics: applications in ophthalmology, APPLIED OPTICS, Vol. 19, No. 11 (1 Jun. 1980), pp. 1749-1757.
A scanning laser ophthalmoscope is a large, expensive piece of equipment intended for use by ophthalmologists or other eye specialists in their offices or laboratories. Equipment of this nature is not suitable for use by non-specialists or emergency personnel in the field. To date, alternating current power sources and complex synchronization circuitry have been necessary to produce and acquire images, and control or ancillary computer systems are typically necessary. This presents a problem with weight, footprint, and power. European and US configurations have had to be designed in, due to 110 as opposed to 220 V power, and 60 as opposed to 50 Hz. Further, the video standards differ from country to country, so that video rate imaging has been made more difficult and cumbersome by requiring two sets of circuitry. Personnel in the field rely on either an ophthalmoscope or fundus camera instrumentation to obtain a view of the human retina. Both direct and indirect ophthalmoscopes require considerable skill to obtain a view of the macula of each eye, often not possessed by medical personnel not trained in an eye subspecialty. These images are not seen by anyone remote from the patient, and therefore a description by unskilled personnel is often the only data available to be transferred to an expert located elsewhere. Direct and indirect ophthalmoscopes do not produce images that can be stored and examined at another time or location. Fundus camera devices do provide film or digital storage of the area examined; however, fundus camera devices using flood illumination of a wide area of the retina often fail to produce images of high contrast. Untrained personnel often do not produce high quality results. The quality of the image of the retina from both fundus cameras and ophthalmoscopes depends upon the diameter of the pupil, with a large diameter pupil needed for acceptable images in the indirect ophthalmoscope and many fundus cameras. Non-mydriatic fundus cameras, requiring little or no dilation, depend upon flashed illumination and relatively expensive, high sensitivity detector arrays to obtain an image. These devices use flood illumination and do not depend upon the light efficiency of a scanning design, and therefore typically operate using uncomfortably bright lights that lead to pupil constriction in eyes not having received dilatation medication.
Those instruments developed commercially to date for digital imaging of the retina require computer systems for operation, including proprietary software. There is considerable training needed by the non-expert to be able to operate such a device. There is often a high degree of computer skill needed to acquire and transmit the image data from these devices. The data are often stored in proprietary file formats or require image storage, processing, or transmission methods that require additional software, which may not be compatible with other patient data. Additional training is necessary to interpret these images, or the data from them, often requiring consultation time and travel of an individual to the site of the instrument for training.
Current imaging devices are not readily suited for use with personal digital assistants, wireless transmission, or the ubiquitous storage devices used with consumer digital cameras. Such images, when using proprietary software or file formats, are not readily transferred and used with laptop or other computers in a shared environment.